1. Field of the Invention
This invention relates generally to a passive millimeter-wave imaging system and, more particularly, to a passive millimeter-wave imaging system that provides a full 360.degree. instantaneous field-of-view by utilizing a spherical Luneburg lens and a thin ring of millimeter-wave direct detection receivers positioned around the lens.
2. Discussion of the Related Art
Imaging systems that generate images of a scene by detecting background millimeter-wave radiation (30-300 GHz) given off by objects in the scene offer significant advantages over other types of imaging systems that provide imaging by detecting visible light, infrared radiation, and other electro-optical radiation. These advantages generally relate to the fact that millimeter-wave radiation can penetrate low visibility and obscured atmospheric conditions caused by many factors, such as clouds, fog, haze, rain, dust, smoke, sandstorms, etc., without significant attenuation, as would occur with the other types of radiation mentioned above. More particularly, certain propagation windows in the millimeter-wavelength spectrum, such as W-Band wavelengths at about 89 to 94 GHz, are not significantly attenuated by the oxygen and water vapor in air. Millimeter-wave radiation is also effective in passing through certain hard substances, such as wood and drywall, to provide imaging capabilities through walls. Thus, millimeter-wave imaging systems are desirable for many applications, such as aircraft landing, collision avoidance and detection systems, detection and tracking systems, surveillance systems, etc. Virtually any type of imaging system that can benefit by providing quality images under low visibility conditions could benefit by using millimeter-wave imaging.
Recent millimeter-wave imaging systems also can offer the advantage of direct detection. This advantage has to do with the fact the millimeter-wave receivers can include components that amplify, filter and detect the actual millimeter-wavelength signals. Other types of imaging system receivers, such as heterodyne receivers, generally convert the received radiation from the scene to intermediate frequencies prior to detection. Therefore, direct detection millimeter-wave receivers that detect the millimeter-wave radiation do not suffer from the typical bandwidth and noise constraints resulting from frequency conversion and do not include the components needed for frequency conversion.
Millimeter-wave imaging systems that use a focal plane imaging array to detect the millimeter-wave radiation and image a scene are known in the art. In these types of systems, the individual receivers that make up the array each includes its own millimeter-wave antenna and detector. An array interface multiplexer is provided that multiplexes the electrical signals from each of the receivers to a processing system. A millimeter-wave focal plane imaging array of this type is disclosed in U.S. Pat. No. 5,438,336 issued to Lee et al., titled "Focal plane Imaging Array With Internal Calibration Source." In this patent, an optical lens focuses millimeter-wave radiation collected from a scene onto an array of pixel element receivers positioned in the focal plane of the lens. Each pixel element receiver includes an antenna that receives the millimeter-wave radiation, a low noise amplifier that amplifies the received millimeter-wave signal, a bandpass filter that filters the received signal to only pass millimeter-wave radiation of a predetermined wavelength, and a diode integration detector that detects the millimeter-wave radiation and generates an electrical signal. The signal from each of the diode detectors is then sent to an array interface unit that multiplexes the electrical signals to a central processing unit to be displayed on a suitable display unit. Each pixel element receiver includes a calibration circuit to provide a background reference signal to the detector. Other types of focal plane imaging arrays including separate detecting pixel elements are also known in the art.
The millimeter-wave imaging systems known in the art typically have a finite field-of-view (FOV) that is limited to a certain angular range, for example 30.degree., relative to the imaging system. However, certain applications, for example, surveillance and reconnaissance or search and tracking applications, generally require a full 360.degree. field-of-view (IFOV) imaging capability where each point around the system is imaged substantially simultaneously. Infrared search and track (IRST) systems are known in the art that provide this type of field-of-view capability. The IRST systems provide the 360.degree. field-of-view by quickly rotating a scanning element. Because passive millimeter-wave imaging systems tend to be larger and bulkier compared with visible light and infrared imaging systems, 360.degree. field-of-view systems have heretofore not been capable in the millimeter-wave environment.
What is needed is a millimeter-wave imaging system that provides a full 360.degree. instantaneous field-of-view (IFOV) imaging. It is therefore an object of the present invention to provide such as imaging system.
Although the present invention focuses on passive millimeter-wave imaging (also known as radiometric imaging), its concept is applicable to all frequencies of the electromagnetic spectrum, from the lower radio frequencies, to the microwave frequencies, to submillimeter wave frequencies, and higher frequencies. It is also applicable to both active (radar) and passive (radiometric) systems.